This invention relates to a method for the preparation of 3,5-dichlorophthalic acid or salt thereof or 3,5-dichlorophthalic anhydride from N-substituted tetrachlorophthalimides and trichlorophthalimides. 3,5-Dichlorophthalic acid is useful in the synthesis of plant growth regulators (U.S. Pat. Nos. 3,940,419 and 4,017,299). 3,5-Dichlorophthalic acid and phthalic anhydride may be prepared in poor yield by direct chlorination methods. For example, phthalic anhydride may be chlorinated to produce a mixture of chlorinated phthalic anhydrides. The products may be separated and, eventually, 3,5-dichlorophthalic anhydride may be obtained in low yield. Hydrolysis converts the anhydride to the acid.
O'Reilly, et. al. disclose in U.S. patent application, Ser. No. 07/439,227, now U.S. Pat. No. 4,981,999 that tetrachloro, trichloro or dichloro phthalic anhydrides may be treated with zinc and sodium hydroxide in aqueous solution in order to remove chlorines. The chlorines are removed in the order, the fifth position first, the fourth position second and the third position third. Thus, if two chlorines are removed from tetrachlorophthalic anhydride the product obtained, after acidification, is 3,6 dichlorophthalic acid. Similarly, if one chlorine is removed from 3,4,6-trichlorophthalic anhydride, the product is 3,6-dichlorophthalic acid. This process involves the removal of chlorine and not an isomerization. The numbering of the dichloro and the trichloro derivatives are different because under the standard system of nomenclature, the trichloro derivative is numbered so as to give the chlorines the lowest possible number.